Field of the Invention
The present invention relates to a liquid crystal drive apparatus configured to drive a liquid crystal element by a digital driving method.
Description of the Related Art
Liquid crystal elements include transmissive liquid crystal elements such as a TN (Twisted Nematic) element and reflective liquid crystal elements such as a VAN (Vertical Alignment Nematic) element. These liquid crystal elements are driven by an analog drive method and a digital drive method. The analog drive method changes a voltage applied to a liquid crystal layer depending on tones to control lightness (brightness), and the digital drive method binarizes the voltage applied to the liquid crystal layer and changes a voltage application time period to control lightness. As such a digital drive method, a sub-frame drive method temporally divides one frame period into multiple sub-frame periods and controls application (ON) and non-application (OFF) of a predetermined voltage to each pixel to cause the pixel to display its tone.
Description will be made of a typical sub-frame drive method. FIG. 12 illustrates an example of dividing one frame period into multiple sub-frame periods (bit lengths). Numerical values written in the respective sub-frames indicate temporal weights of these sub-frames in the one frame period. The example shows a case of expressing 64 tones. In this example, a sub-frame period having a temporal weight of 1+2+4+8+16 is referred to as “an A sub-frame period”, and a sub-frame period having a temporal weight of 32 is referred to as “a B sub-frame period”. Furthermore, a sub-frame period where the predetermined voltage is applied is referred to as an ON period“, and a sub-frame period where the predetermined voltage is not applied is referred to as an OFF period”.
FIG. 13 illustrates all tone data corresponding to the division example illustrated in FIG. 17. A vertical axis indicates tones, and a horizontal axis indicates one frame period. A white sub-frame period indicates the ON period where the pixel is in a white display state, and a black sub-frame period indicates the OFF period where the pixel is in a black display state. According to these tone data, when two pixels adjacent to each other (hereinafter referred to as “adjacent pixels”) in a liquid crystal element display two tones adjacent to each other (hereinafter referred to as “adjacent tones”) such as 32 and 33 tones, the 32 tone is displayed by setting the A sub-frame period to the ON period and setting the B sub-frame period to the OFF period, and the 33 tone is displayed by setting the A sub-frame period to the OFF period and setting the B sub-frame period to the ON period.
Such a state where the ON and OFF periods temporally overlap each other in the adjacent pixels, that is, the predetermined voltage is applied to one (ON-period pixel) of the adjacent pixels and the predetermined voltage is not applied to the other one (OFF-period pixel) of the adjacent pixels generates so-called disclination, which generates a decrease in lightness of the ON-period pixel. FIG. 14 illustrates an example of the decrease in lightness due to the disclination. FIG. 19 illustrates tones in its vertical direction, and its contrasting density illustrates displayed lightness. When the disclination is not generated, a smooth contrasting density can be expressed. However, when the adjacent pixels display two adjacent tones (such as the 32 and 33 tones) corresponding to a case where the ON and OFF periods overlap each other for a long time, the displayed lightness is decreased due to the disclination, which generates a dark line.
Japanese Patent Laid-Open No. 2013-050681 discloses a drive circuit that divides one or more long sub-frame periods into periods each equal to a short sub-frame period to produce multiple divided sub-frame periods. The drive circuit disclosed in Japanese Patent Laid-Open No. 2013-050681 performs, when phases of bits of tone data corresponding to adjacent pixels are mutually different, a process to maintain their tones and corrects a bit arrangement of the tone data corresponding to one of the adjacent pixels so as to make it closer to a bit arrangement of the tone data corresponding to the other one of the adjacent pixels. This process enables, compared with a case of not dividing the long sub-frame period, shortening the sub-frame period (hereinafter referred to as “an ON/OFF adjacent period”) where the ON and OFF periods mutually overlap between the adjacent pixels.
However, in the method disclosed in Japanese Patent Laid-Open No. 2013-050681, a shortest ON/OFF adjacent period of the adjacent pixels is too long to ignore the decrease in lightness due to the disclination. Furthermore, in the method, a long ON/OFF adjacent period of the adjacent pixels increases an amount of the decrease in lightness due to the disclination depending on a response speed of liquid crystal molecules.
FIG. 15 illustrates all tone data disclosed in Japanese Patent Laid-Open No. 2013-050681 where an A sub-frame corresponds to a temporal weight of 1+2+4+8 and a B sub-frame is divided into multiple divided sub-frame periods 1SF (SF means a sub-frame) to 10SF each corresponding to a temporal weight of 8. One divided sub-frame period is 0.39 ms. In the tone data, the shortest ON/OFF adjacent period of the adjacent pixels is 1.39 ms that corresponds to two divided sub-frame period. Thus, the decrease in lightness (that is, the dark line) due to the disclination is noticeable.